This invention relates to a thermoplastic resin composition having low linear expansion coefficient, excellent dimensional stability and an excellent balance between impact resistance and rigidity.
In recent years, for the purpose of reducing fuel consumption by weight saving of automobiles, plastics have been frequently used as parts for automobiles, and various plastic materials have been used not only for interior parts such as an instrument panel, a console box, a glove box, a handle and a trim and exterior parts such as a protection mold, a lamp housing, a front grille, a mud guard and a side bumper, but also for a bumper, a fascia, a fender, a door panel and a part of a body for which metallic materials have been conventionally used.
As plastics used for the above parts for automobiles, there may be mentioned, for example, RIM (reaction injection molding) urethane, composite polypropylene, inorganic substance-reinforced plastics such as glass fiber-reinforced polyamide and polymer alloy materials such as polycarbonate/polybutylene terephthalate (PC/PBT) and polyphenylene ether/polyamide (PPE/PA). Among these materials, as the composite polypropylene, there have been described, for example, polypropylene type compositions in which a partially crosslinked ethylene-propylene copolymer rubber and oil are formulated into polypropylene in Japanese Provisional Patent Publications No. 145857/1978, No. 16554/1979 and No. 135847/1982, and as the polymer alloy of PPE/PA, there has been disclosed, for example, a composition in which a rubber modified with a polar group-containing compound is added as an impact modifier to a combination of polyphenylene ether and polyamide in Japanese Provisional Patent Publication No. 49753/1981. Further, for the purpose of improving a balance between low temperature impact resistance and rigidity, there has been proposed a composition comprising a modified polyphenylene ether intermediate, polyamide and an impact modifier in Japanese Provisional Patent Publication No. 19664/1989.
Further high level characteristics of a material used for a fascia, a fender and a door panel have been particularly demanded as compared with those of conventional plastic parts. There may be mentioned, for example, (1) impact resistance: a characteristic of absorbing energy at the time of crash by distortion and then recovering from distortion and a characteristic of ductile fracture by impact at low temperatures; and (2) dimensional stability by low linear expansion coefficient in order to solve the following problems. The thermal expansion degree of a coating is different from that of a plastic base material so that when a plastic molded product after coating is used under circumstances at high temperatures, the coating is peeled off or fine cracks are formed on a coated surface, whereby appearance and a design are worsened in many cases. Further, when a large-sized plastic molded product is used in combination with a molded product of other material such as wood or metal, the thermal expansion degrees of both molded products are different from each other under circumstances at high temperature so that there have been problems of a dimensional difference and failure of engagement. Thus, it has been demanded to establish techniques of providing a material which can satisfy the characteristics described in the above (1) and (2), i.e., techniques of improving impact resistance of plastics and improving dimensional stability thereof at high temperatures (techniques of controlling thermal expansion coefficient).
In a conventional plastic material for automobiles, it has been difficult to provide dimensional precision (linear expansion coefficient) at high temperatures and high level of impact resistance simultaneously. Among common counter-measures, there has been well known, for example, a method of improving impact resistance by formulating a large amount of an elastomer, but in this method, dimensional precision (linear expansion coefficient) at high temperatures is worsened. As a means of improving impact resistance by formulating a predetermined amount of an elastomer, a means of improving morphology has been generally known. This means is a method of making a domain particle size in a finely dispersed state (domain-matrix structure) finer by enhancing compatibility between different thermoplastic resins by a special blend technique or a special formulation technique, but dimensional precision (linear expansion coefficient) at high temperatures cannot be improved by this method.
Further, for the purpose of improving dimensional precision (linear expansion coefficient), a means of formulating an inorganic filler may be considered. However, in this means, a molded product is liable to be brittle so that the level of impact resistance is lowered, which shows that brittleness causes fracture particularly by impact at low temperature, and its application is extremely limited.